Chloride free leaching process for porous zinc anodes

ABSTRACT

A porous zinc anode produced and adapted for use in alkaline solution, said anode in said solution constituting means for obviating gassing characteristics as contrasted to a porous zinc anode produced in other ways. The essential step in the invention is free acid leaching with an acid other than hydrochloric acid in order to form a water soluble salt of the free acid.

United States Patent 1 1 Curelop et al.

[ 1 Jan. 30, 1973 54] CHLORIDE FREE LEACHING PROCESS FOR POROUSZINCANODES [75] Inventors: Edward J. Curelop, Brockton; Nikola Marincic,Winchester, both of Mass.

731 Assigneez Pl R. Mallory & Co., Inc., Indianapo1is,lnd.

22 Filed: Feb.10, 1971 21 App1.No.:1l4,34 9

[52] US. Cl ..136/30 [51] Int. Cl. ..H0lm 43/02 [58] Field of Search....l36/303l, 125-129,

[56] I References Cited 3 UNITED STATES PATENTS 413,438 10/1889 Eggers..75/169 2,938,064 5/1960 Kordeschi... ....l36/125 X 2,983,777 5/1961Yardney ..136/20 3,007,994 11/1961 Urry ..l36/126 3,060,254 10/1962 Urry..136/24 3,236,690 2/1966 Booe et a1. ..l36/68 3,427,203 2/1969 Fletcher..1 36/120 3,427,204 2/1969 Clune et a1. ....136/l26 2/1971 Jerabek.'...13 6/126 Primary Examiner-Anthony Skapars AttorneyRobert Levine vABSTRACT A porous zinc anode produced and adapted for use in alkalinesolution, said anode in said solution constituting means for obviatinggassing characteristics as contrasted to a porous zinc anode produced inother ways. The essential step in the invention is free acid leachingwith an acid'other than hydrochloric acid in order to form a watersoluble salt of the free acid.

4 Claims, No Drawings CHLORIDE FREE LEACHING PROCESS FOR POROUS ZINCANODES This invention relates to batteries and more particularly tovimproved structures and systems therefore, and specifically to theelectrodes used therein.

The efficiency of a battery depends to great extent on the surface areaof theanode structure used therein. The present invention provides ananode of greatly improved characteristics comprising compacted porouszinc amalgam, and utilizing an aqueous solution leachant of a'chloridefree acid in combination with the salt of this chloride free acid as afillerin contact with this acid. A zinc mercury amalgam is created by anelectrochemical reaction eementation process, and a water soluble saltof the chloride free acid is formed simultaneously.

The reactive cementation process utilizes the electro-chemical reactionof zinc powder with mercury oxide to produce an alloying or amalgam bondbetween the zinc particles. In order to facilitate the process ofamalgamation of the zinc powder, a free acid other than hydrochloricacidis used as afluxing agent for the zinc particles to facilitate theirintimate contact with each other, along with the use of a chloride freefiller that is a soluble salt of the chloride free acid. Examples ofchloride free acids and of their corresponding chloride free fillersinclude acetic acid and sodium acetate, or boric acid and sodium borate,or oxalic acid and sodium oxalate. p

In the past zincelectrodes were produced by preparing a compacted porousmixture of zinc powder, mecurous, chloride, and ammonium chloride; andadding water thereto so as to utilize the ammonium chloride to dissolveand to clean away the zinc oxide on the surface of the zincparticles.The newly cleaned' zinc particles would then ,react withmercurouschloride to liberate the metal mercury from its salt and to form anamalgam bond with the. newly liberated m'ei'cury. M

It was found that gassing may occur from zinc anodes produced by theammonium chloride process, with this instability toward gassing beingdue to the presence of unremoved zinc.oxide and/or zinc chlorideimpurity sites on the zinc anodethat had notbeen displaced by mercurymetal during. the electrochemical displacement reaction.

Furthermore in the-past it was thought that a way to avoid gassing wouldbe to have zinc mercury amalgam spread over substantially the entireanode surface. Nevertheless anode gassing stillmay result even with acomplete amalgamcoating. it has been found here that if a chloride freeacid leaching'agent is used in combination with a chloride free fillermaterial to dissolve zinc oxide on the surface of zincparticles tofacilitate their intimate; metal tometal contact with a one another andwith'metallic mercury, and alsoto deposit trace amounts of chloride freeinert ions integrally united to the zinc at specific lattice sites; thenit is possible to prevent or to substantially reduce gassing at thosesites caused by the zinc oxide or the zinc chloride that would otherwisebe present therein. These ions'are inert because they do not directlyorindirectly cause anode gassing to occur. 4 r

It is therefore a prime object of the present invention to provide, in-abattery,an amalgamated zinc anode of great porosity andfuniform densityfabricated by reactive cementation and characterized by substantiallyreduced gassing in the finished anode.

It is another object of the present invention to provide finished anodesshowing much lower gassing rates in alkaline solutions than do anodesproduced by a process using chloride ions; having much cleaner surfaceswith an appearance of a sparkling crystalline structure completely freeof zinc oxide; and generating much higher short circuit discharge ratesdue to a much larger active clean surface area present in the porousanode structure.

It is another object of the present invention to provide an anodestructure for an electrochemical cell, said structure formed by mixingspecific portions of zinc powder, mercuric oxide, and a chloride freefiller selected from the group consisting of sodium acetate, sodiumborate and sodium oxalate, and by leaching out said chloride free fillerwith an aqueous solution of the corresponding chloride free acidselected from the group consisting of acetic acid, boric acid and oxalicacid.

It is a further object of the present invention to produce va porousanode structure of specific shape in the form of a selfsupporting openthree dimensional zinc mercury metal network consisting essentially ofchloride free zinc and mercury amalgam metal particles which arepressure welded one to another having communicating'voids therebetweenand further consisting, of trace amounts of inert nonchloride ionsselected from the group consisting of acetate ions, borate ions,oxalateions present therein up to 1 percent by weight of said structureintegrally united at specific lattice sites to reduce anode gassing.

Still another object of the present invention is to provide aprimary'cell including azinc amalgam porous anode for a battery.

Other objects of. the invention will. become apparent fromthe followingdescription.

Generallyspeaking, the present invention providesa new battery system inwhich the problem of anode gassing is substantially reduced. Thissubstantial reduction is the direct result of the presence, in thebattery, of a novel anode consisting of porous zinc amalgam containingtrace amounts of an inert nonchloride ion selected from the groupconsisting of acetate ions, borate ions and oxalate'ions present up to1% by weight of the anode. The anode, is forr ned as a uniformly porouspellet by means of reactive cementationof admixed materials such'as zincpowder, mercuric oxide,

and a chloride free .filler selected from the'group con sisting ofsodium acetate, sodium borate,'and sodium oxalate, and by leaching outthe chloride free filler with an aqueous solution of the correspondingchloride free acid selected from the group consisting of acetic acid,

1 boricacid and oxalic acid."

The blending was carried out for, 10-12 minutes at speed of -30 r.p.m.About 1.68 to 1.70 grams of the above mixture was pressed into a greenanode 77-80 mils thick. The pressure required was about 12,000 psi.

The leaching of the anodes was carried out with acetic acid diluted inthe ratio of l part by weight acid to 3 parts by weight water. Theamounts of acid should be in a controlled-relation to the weight andnumber of anodes, at least during the first 15 minutes of leaching.About 2 ml. of the above solution is the maximum allowed for the aboveanodes; in other words approximately 1 ml. of the above solution pereach gram of the green anode of anysize and shape. The excess of theabove leaching solution can be added for faster leaching but only afterthe initial 15 minute period.

The acetate leaching process is generally faster than the chlorideleaching process. It is completed in 5-6 hours at room temperature whenthe above size anodes are processed. The leached anodes were washed indistilled water, until the effluent showed a pH value of 6 or higher.The anodes were then washed with alcohol, dried in air, and had a dryweight of 1.14 grams i 0.05 grams and had a porosity of 70 percent andcontained 12 percent mercury.

EXAMPLE 2 Anodes of various porosities can be produced using the sameprocedure except for the mix composition. The following is anothervariation of the above mix, used for the productionof large anodes: 5000grams of zinc powder, 1300 grams of sodium acetate, 780 grams H,O, and30 ml. kerosene. i 48.5 grams of the abovemix were pressed into aretangular anode 3.2 inches by 1.6 inches with a force of 30 tons (5.86tons per square inch). The resulting anode produced by the chloride freeacetic acid leaching process'weighed 36.5 grams and contained 12 percentmercury; it was 4.2 mm. thick and 64 porous. The same type of anode waspressed with a copper screen in the middle as a current collector forthe rechargeable cell application. The copper screen was amalgamatedduring the leaching process and provided a good contact to the porousanode body over the entire anode cross-section.

EXAMPLE 3 Other combinations of acids as leachants and their salts asfillers were also utilized. The following mixture of 5000 grams zincpowder, 2100 grams sodium metaborate, 780 grams 11,0 was prepared andthe pressed anodes were. leached inacorr'e'spondingly diluted boric acidin the ratio ofl ml. per gram of green anode as in Example l andproduced a zinc anode of about the same weight, porosity,' and mercurycontent of Example 1.

EXAMPLE 4 The-following mixture of 5000 grams zinc powder, l300gramssodiumkmetaborate, 780 grams H O,,and 30 ml. kerosene was treated withthesame quantity of boric acid as in Example .3 and produced a zincanode of about the same weight porosity, and mercury content of Example2. r

EXAMPLE 5 The following mixture of 5000 grams zinc powder, 2100 gramssodium oxalate, 780 grams H,O, and 30 ml. kerosene was treated withcorrespondingly diluted oxalic acid in the ratio of 1 ml. per gram ofgreen anode as in Example 1 and produced a zinc anode of about the sameweight porosity, and mercury content of Example 1.

EXAMPLE 6 The following mixture of 5000 grams zinc powder, 1300 gramssodium oxalate, 780 grams 11,0, and 30 ml. kerosene was treated with thesame quantity of oxalic acid as in Example 5 and produced a zinc anodeof about the same weight, porosity, and mercury content of Example 2.

Zinc particles in anodes produced by the above 6 examples measuredbetween 25 to 40 microns.

A" zinc anode prepared according to each of the above six examples wastested as follows. The anode, wherever necessary, was reshaped into theform of a disc pellet having a diameter of v 0.5 inches and a thicknessof 0.1 inches. The pelletwas then inserted as an anode into an alkalinecell/of flat cylindrical construction .in contact with an absorbentspacer impregnated with an alkaline electrolyte, such as 35-40% KOH,3.5-6.5% ZnO, and the balance water. In contact with the oppositesurface of said spacer was a suitable depolarizer such as the metaloxide H O, M,,O or Ag O containing from 5-25 percent graphite. Thissealed cell was subjected to F for 24 hours with the result that about0.05-0.07 cu. cm. of gas were produced.

When zinc pellets of about the same size weight, porosity, and mercurycontent were produced by the use of ammonium chloride, the gassing ratewas determined to be 0.20 cu. cm. to 0.25 cu. cm. for a 24 hour periodat 180F.

Thus the gassing rates for the chloride free leaching are verysatisfactory relative to the gassing rate for the chloride process andindicate the superiority of the present invention over conventionalanode structures.

Although the present invention has been disclosed in connection with afew preferred embodiments thereof, variations and modifications may beresorted to by those skilled in the art without departing from theprinciples of the new invention. All of these variations andmodifications are considered to be within the true spirit and scope ofthe present invention as disclosed in the foregoing description anddefined by the appended claims. v

What is claimed is:

1. A method of fabricating a highly porous amalgamated chloride freezinc anode for an electric current producing devicecomprising the stepsof: preparing a reactive mixture: consisting of zinc powder, mercuricoxide powder, and a chloride free filler material selected from thegroupconsisting of sodium acetate, sodium borate and sodium oxalate andleaching said mixture with a chloride free acid leachant selected fromthe group consisting of acetic acid, boric acid and oxalic acid fordissolving out said filler material to produce a mercury zinc amalgam.

lattice sites to substantially reduce anode gassing.

4. The method of claim 3 wherein the trace amounts of inert ions arepresent up to 1 percent by weight of the amalgam; and wherein thechloride free amalgam has a substantially reduced gassing rate of 0.05to 0.07 cubic centimeters per 24 hours.

1. A method of fabricating a highly porous amalgamated chloride freezinc anode for an electric current producing device comprising the stepsof: preparing a reactive mixture consisting of zinc powder, mercuricoxide powder, and a chloride free filler material selected from thegroup consisting of sodium acetate, sodium borate and sodium oxalate andleaching said mixture with a chloride free acid leachant selected fromthe group consisting of acetic acid, boric acid and oxalic acid fordissolving out said filler material to produce a mercury zinc amalgam.2. The method of claim 1 further comprising the steps of: washing saidamalgam with distilled water until the effluent has a pH of at least 6;washing said amalgam with alcohol; and drying said amalgam.
 3. Themethod of claim 2 wherein said chloride free amalgam contains traceamounts of inert chloride free ions selected from the group consistingof acetate ions, borate ions and oxalate ions integrally united atspecific lattice sites to substantially reduce anode gassing.